1. Field of the Invention
The present invention generally relates to the manufacture of transistors for integrated circuits and, more particularly, to the production of complementary pairs of field effect transistors of enhanced performance at extremely small scale.
2. Description of the Prior Art
Performance and economic factors of integrated circuit design and manufacture have caused the scale of elements (e.g. transistors, capacitors and the like) of integrated circuits to be drastically reduced in size and increased in proximity on a chip. That is, increased integration density and proximity of elements reduces the signal propagation path length and reduces signal propagation time and susceptibility to noise and increase of possible clock rates while the reduction in element size necessary for increased integration density increases the ratio of functionality which can be provided on a chip to the costs of production (e.g. wafer/chip area and process materials) per chip and, potentially, the cost of devices containing the chips by reducing the number of inter-chip and inter-board connections required.
However, the immutable material properties and physical effects by which transistors and other elements function is invariably compromised as the scale of integrated circuit elements is reduced. In response, many improvements in transistor design have been made to maintain suitable levels of performance of these elements. For example, lightly doped drain (LDD) structures (now generally referred to as extension implants since heavier doping levels have been required in current minimum feature size regimes), halo implants and graded impurity profiles have been employed to counteract short channel and punch-through effects and the like, particularly in field effect transistors (FETs) which have become the active device of choice for all but the highest frequency devices. Reduction in device scale has also required operation at reduced voltages to maintain adequate performance without device damage even though operating margins may be reduced.
A principal factor in maintaining adequate performance in field effect transistors is carrier mobility which affects the amount of current or charge which may flow (as electrons or holes) in a doped semiconductor channel under control of a voltage placed on a gate electrode insulated from the channel by a very thin dielectric. Reduced carrier mobility in an FET reduces not only the switching speed/slew rate of a given transistor but also reduces the difference between “on” resistance to “off” resistance. This latter effect increases susceptibility to noise and reduces the number of and/or speed at which downstream transistor gates (capacitive loads) can be driven, sometimes referred to as fan-out. Even during the early development of metal-oxide-semiconductor (MOS) field effect transistors and complementary MOS (CMOS) devices (widely used in integrated circuits at the present time), in particular, carrier mobility was a design concern and often required a pMOS device to be made approximately two times as large as a complementary nMOS device with which it was paired in order to obtain reasonably symmetrical operation of the CMOS pair in view of the difference in carrier mobility between electrons, the principal carrier in nMOS devices and holes, the principal carrier in pMOS devices. In more recent and critical designs, it has been shown that carrier mobility degrades in deeply scaled bulk MOS devices due to the heavy doping required to suppress short-channel effects and ultra-thin oxide effects.
It has also been shown theoretically and confirmed experimentally that mechanical stress in the channel region of an FET can increase or decrease carrier mobility significantly; depending on the sign of the stress (e.g. tensile or compressive) and the carrier type (e.g. electron or hole). Tensile stress increases electron mobility and decreases hole mobility while compressive stress increases hole mobility while decreasing electron mobility in the doped semiconductor crystal lattice forming the transistor channel. This phenomenon is well-recognized and theories concerning the physical effects by which it occurs are, in any event, unimportant to its exploitation. In this regard, numerous structures and materials have been proposed for inducing tensile or compressive force in a semiconductor material, such as shallow trench isolation (STI) structures, gate spacers, etch-stop layers and silicide which are generally included in integrated circuit designs. It is also known to induce persistent stresses in semi-conductors for structural reasons. For example, U.S. Pat. Nos. 6,069,049 and 6,399,976 teach that applying a film around a structure and then reducing volume of the film to compress the structure and thus prevent the propagation of defects. However, at the present state of the art, structures for producing stresses can generally be made of only one type; to produce tensile stress or compressive stress but not both. Therefore, in integrated circuit designs using both pFET and nFET transistors and CMOS technology (in which the logic is implemented principally by complementary pMOS and nMOS transistor pairs), in particular, an enhancement of carrier mobility in one type of transistor was necessarily accompanied by degradation of carrier mobility in the other or complementary type of transistor; yielding little, if any, net performance gain, although symmetry of operation could theoretically be improved. Moreover, stress of a single type produced by such structures tends to cause warping or curling of the wafer or substrate (particularly if thin, as is a current trend) which compromises later lithographic processes such as the formation of contacts and connections or, in severe or high stress cases, chip or wafer cracking; reducing manufacturing yield or (in rare cases) reliability after being put into service. Further, the stress levels produced by such structures were generally difficult to control particularly since the structure dimensions are often dictated by other design concerns, such as isolation and breakdown voltages.